Day 174 — Listening: Why Some Concert Halls Sound Better — Section 4（講義）

Good morning, everyone. Last week we looked at how sound travels outdoors. Today I want to move indoors and ask a question that architects struggled with for centuries: why do some concert halls sound wonderful, while others, often far more expensive, sound disappointing?

Let me begin at Harvard in 1895. The university had just opened a lecture hall in its new art museum, the Fogg, and almost at once there were complaints. Students simply couldn't follow what the lecturer was saying, because every word seemed to hang in the air. The senior physicists regarded the problem as impossible, so it was handed to a young assistant professor, Wallace Sabine. Now, you might expect a physicist to start with equations. Sabine didn't. He started with cushions. Late at night, when the campus was quiet, he and his assistants carried hundreds of seat cushions across from a nearby theatre, added them to the hall a few at a time, and timed how long a single note could still be heard after it stopped.

What Sabine was measuring is what we now call reverberation time. Formally, it's the number of seconds it takes for a sound to fade by sixty decibels, which is roughly the drop from a loud chord to near silence. Be careful here: the sixty decibels is just the agreed amount of fading; the thing you actually measure is time. And what he found was surprisingly simple. Reverberation time depends on two things: the volume of the room and the total amount of absorbing material in it. Make the room bigger and the sound lasts longer. Add soft surfaces, such as curtains, cushions, or an audience in winter coats, and it dies away sooner. The shape of the room, which most architects of the day believed was the decisive factor, didn't appear in his formula at all.

Five years later, in 1900, Sabine was able to test his ideas on a much larger scale, as the acoustical consultant for Boston's new Symphony Hall. You'll often read that it was the first hall designed entirely according to science. Personally, I think that's an exaggeration. The architects modelled it closely on a successful hall in Leipzig, so Sabine was adapting a proven design rather than inventing a new one. Even so, the result is regularly ranked alongside the Musikverein in Vienna and the Concertgebouw in Amsterdam as one of the three finest concert halls in the world. And all three share a shape: a tall, narrow rectangle, which acousticians affectionately call the shoebox.

So why does the shoebox work so well? The obvious answer would be reverberation time, and it's true that all three have a fairly long one, around two seconds when full. But plenty of halls with a two-second reverberation time sound flat. The more important factor turned out to be the side walls. Because the room is narrow, sound bounces off the walls and reaches each listener from the left and the right a fraction of a second after the direct sound. Our two ears receive slightly different signals, and the brain interprets this as the music surrounding us. Acousticians call this quality envelopment, and in listening tests it seems to matter more to people than reverberation time alone.

Which brings me to the most famous failure in modern acoustics. In 1962, New York opened Philharmonic Hall at Lincoln Center. It was wide, it was modern, and it held more people than the old shoeboxes. Reviews were mixed from the first night. Musicians complained that they couldn't hear each other on stage, and audiences described the sound as thin. Admittedly, some conductors praised the hall, and the press may have enjoyed the controversy a little too much. But the building tells its own story. It was rebuilt in 1976, and then renovated again, at a cost of five hundred and fifty million dollars, before reopening in 2022.

Does that mean every new hall should be a shoebox? Not necessarily. In 1963, Berlin opened its Philharmonie, where the orchestra sits in the centre and the audience surrounds it on terraces, rather like a vineyard on a hillside. Each terrace is edged by its own low walls, and these do the job that the side walls do in a shoebox, sending reflections in from the side. Because vineyard halls bring more of the audience close to the stage, many recent halls have followed that model. The trade-off is that someone sitting behind the orchestra hears the brass clearly but a singer's voice poorly, since the voice is directed away from them.

So, to sum up: a good hall is less a matter of size or style than of where the reflections come from. Next week, we'll look at how computer modelling now lets acousticians test those reflections before a single brick is laid.